Seat belt apparatuses, which have been installed in vehicles such as automobiles, restrain an occupant with a seat belt in an emergency such as a vehicle collision, thereby preventing the occupant from being thrown from their seat. Such seat belt apparatuses have a seat belt retractor that retracts the seat belt. In this seat belt retractor, the seat belt is retracted onto a spool when the seat belt is not fastened, and the seat belt is withdrawn and fastened by the occupant when the seat belt is fastened. In the above emergency, a lock mechanism of the seat belt retractor is activated and stops the rotation of the spool in the belt withdrawing direction, thereby preventing the seat belt from being withdrawn. Thus, the occupant is restrained by the seat belt in the emergency.
In conventional seat belt apparatuses, various belt tension modes are set according to the running state of the vehicle, the usage state of the seat belt apparatus, and the like. Many seat belt apparatuses are known that have, as a seat belt retractor, a motor retractor that rotates a seat belt retracting spool with the power of a motor. In this seat belt retractor, a controller controls an electric motor that is a driving means and thereby controls the belt retraction and belt withdrawal of the spool so that the belt is subjected to the belt tension of the belt tension mode set according to the running state of the vehicle, the usage state of the seat belt apparatus, and the like.
In the meantime, in order for the controller to control the belt retraction and belt withdrawal of the spool by controlling the driving of the electric motor, it is necessary to detect the rotation amount (rotational position) and rotation direction of the spool. So, there has been proposed a seat belt retractor having a rotation sensor detecting the rotation amount and rotation direction of a spool, the rotation sensor including: a rotating disk supported by the rotating shaft of the spool rotatably integrally with the spool; a magnet; and a Hall element (Hall IC) detecting the rotation of the rotating disk by detecting the magnet, the seat belt retractor controlling an electric motor on the basis of the rotation amount of the spool detected by the rotation sensor (see, for example, Japanese Unexamined Patent Application Publication No. 2009-113718).
FIG. 8 shows a seat belt apparatus described in Japanese Unexamined Patent Application Publication No. 2009-113718. FIG. 9 is a partial sectional view of a seat belt retractor described in Japanese Unexamined Patent Application Publication No. 2009-113718. FIG. 10A shows a ring-shaped magnet of a rotation sensor described in Japanese Unexamined Patent Application Publication No. 2009-113718. FIG. 10B is a partial view corresponding to part XB in FIG. 10A. FIG. 10C is a sectional view taken along line XC-XC in FIG. 10B. FIG. 11 illustrates the detection of rotation by the rotation sensor.
In FIGS. 8 to 11, reference numeral 1 denotes a seat belt apparatus; reference numeral 2 denotes a vehicle seat; reference numeral 3 denotes a seat belt retractor that is a motor retractor; reference numeral 4 denotes a seat belt that is withdrawably retracted by the seat belt retractor 3 and has a belt anchor 4a at one end thereof that is fixed to the floor of the vehicle body or the vehicle seat 2; reference numeral 5 denotes a guide anchor that guides the seat belt 4 withdrawn from the seat belt retractor 3 to the shoulder of an occupant; reference numeral 6 denotes a tongue that is slidably supported by the seat belt 4 guided by the guide anchor 5; reference numeral 7 denotes a buckle that is fixed to the floor of the vehicle body or the vehicle seat and into which the tongue 6 is inserted and releasably engaged; reference numeral 8 denotes an electric motor that is a driving means that rotates the spool of the seat belt retractor 3 and thereby retracts and withdraws the seat belt 4; reference numeral 9 denotes a U-shaped frame; reference numeral 9a denotes the left side wall in FIG. 9 of the frame 9; reference numeral 9b denotes the right side wall in FIG. 9 of the frame 9; reference numeral 10 denotes a spool; reference numeral 10a denotes the rotating shaft of the spool 10; reference numeral 11 denotes a locking mechanism; reference numeral 12 denotes a deceleration sensing mechanism; reference numeral 13 denotes a spring mechanism that urges the spool in the retracting direction; reference numeral 13a denotes a case of the spring mechanism 13; reference numeral 14 denotes a power transmission mechanism, for example, a planetary gear deceleration mechanism or an external gear deceleration mechanism; reference numeral 14a denotes a case of the power transmission mechanism 14; reference numeral 15 denotes a rotation sensor that is a rotation amount detector; reference numeral 16 denotes a controller (CPU); reference numeral 17 denotes a pretensioner; reference numeral 18 denotes a rotating disk of the rotation sensor 15 attached to the rotating shaft 10a of the spool 10 with a bushing 10b rotatably integrally with the rotating shaft 10a and concentrically with the rotating shaft 10a; reference numeral 19 denotes a bracket fixed to the right side wall 9b of the frame 9; reference numerals 20a and 20b denote a pair of first and second Hall elements (first and second Hall ICs), respectively, that are disposed with a predetermined interval therebetween in the circumferential direction of a circle concentric with the rotating shaft 10a, attached to the bracket 19, and electrically connected to the controller 16; reference numeral 21 denotes a ring-shaped magnet of the rotating disk 18 that is concentric with the rotating shaft 10a and in which N-pole magnets 21a and S-pole magnets 21b are arranged alternately; and reference numeral 22 denotes a ring-shaped magnet holding member of the rotating disk 18 that holds the magnet 21 and is attached to the rotating shaft 10a of the spool 10 rotatably integrally with the spool 10 and concentrically with the rotating shaft 10a. 
When the spool 10 that is a moving member (rotating member) rotates in the seat belt withdrawing direction, the rotating disk 18, that is, the magnet 21 also rotates in the seat belt withdrawing direction in conjunction with the rotation of the spool 10. The first and second Hall ICs 20a and 20b detect the magnetic poles of the N-pole magnets 21a and the magnetic poles of the S-pole magnets 21b, respectively, and output their detection signals to the controller 16. At this time, the first and second Hall ICs 20a and 20b detect the N-pole magnets 21a and the S-pole magnets 21b alternately. Therefore, the current polarities of the detection signals of the first and second Hall ICs 20a and 20b switch, and the phases of the detection signals of the first and second Hall ICs 20a and 20b differ by a predetermined amount. The controller 16 detects the rotation amount (rotational position) of the spool 10 by counting the number of times of switching of the current polarities of the detection signals from the first and second Hall ICs 20a and 20b. On the basis of the phase difference between the detection signals from the first and second Hall ICs 20a and 20b, the controller 16 determines whether the rotation direction of the spool 10 is the seat belt withdrawing direction or the seat belt retracting direction. On the basis of the rotation amount of the spool 10 and the rotation direction of the spool 10, the controller 16 controls the driving of the electric motor 8 and thereby controls the belt tension of the seat belt 4.
As described above, the rotation sensor 15 that detects the rotation amount of the spool 10 and the rotation direction of the spool 10 serves as a position detector that detects the position and moving direction of a moving member.
In a position detector such as that described above that detects the magnetisms of alternately-arranged N-pole magnets and S-pole magnets using magnetic detecting members such as Hall elements, the magnetization width of the N-pole magnets and the magnetization width of the S-pole magnets are set equal to each other (see, for example, Japanese Unexamined Patent Application Publication No. 2009-113718 and Japanese Unexamined Patent Application Publication No. 2004-264136).
In the meantime, magnetic detecting members such as Hall elements include unipolar detection type magnetic detecting members that detect only the N-pole or S-pole. However, when such unipolar detection type magnetic detecting members are used to detect the N-pole magnets and S-pole magnets that are arranged alternately and have equal magnetization widths and to output the position of the moving member, there are the following problems.
FIGS. 12A and 12B illustrate the problems in the case where unipolar detection type magnetic detecting members are used to detect the N-pole magnets and S-pole magnets that are arranged alternately and have equal magnetization widths. In FIGS. 12A and 12B, the same reference numerals will be used to designate the same components as those shown in FIG. 10.
Suppose that, as shown in FIG. 12A, a rotation sensor 15 has: a rotatable ring-shaped magnet 21 that has six N-pole magnets and six S-pole magnets arranged alternately and having the same magnetization width of 30° in the circumferential direction of the magnet 21; and a pair of first and second Hall ICs 20a and 20B of unipolar detection type detecting only the S-pole and disposed so that their magnetic pole detection points are 45° apart in the circumferential direction of the magnet 21. Suppose that when the rotation angle of the magnet 21 is 0°, the magnet pole detection point (magnetization position) of the first Hall IC 20a is located at the border between an N-pole magnet 21a and an S-pole magnet 21b shown in FIG. 12A. At this time, the magnetic pole detection point (magnetization position) of the second Hall IC 20b is located at the middle position of the magnetization width in the circumferential direction of an N-pole magnet 21a shown in FIG. 2A.
Suppose that the magnet 21 rotates in the β direction (clockwise in FIG. 12A). The magnetic flux density (mT) due to the magnet 21 at the pole detection point of the first Hall IC 20a becomes a sine curve shown by solid line in FIG. 12(b). The magnetic flux density (mT) due to the magnet 21 at the pole detection point of the second Hall IC 20b becomes a sine curve shown by dotted line in FIG. 12(b). The first Hall IC 20a outputs a detection signal (ON) at an S-pole of a magnetic flux density (mT) of a predetermined value or more but does not output a detection signal (OFF) at a magnetic pole other than this. That is, the first Hall IC 20a outputs a detection signal at rotation angles of the magnet 21 between 5° and 25°, and does not output a detection signal at rotation angles of the magnet 21 between 25° and 65°. After that, with the rotation of the magnet 21, the first Hall IC 20a periodically repeats ON and OFF. Similarly, the second Hall IC 20b outputs a detection signal at rotation angles of the magnet 21 between 20° and 40°, and does not output a detection signal at rotation angles of the magnet 21 between 40° and 80°. After that, with the rotation of the magnet 21, the second Hall IC 20b periodically repeats ON and OFF.
As described above, the rotation angle of the magnet 21 at which the first and second Hall ICs 20a and 20b output detection signals is 20°, and the rotation angle of the magnet 21 at which the first and second Hall ICs 20a and 20b do not output detection signals is 40°. Therefore, the range of detection of rotation angle by the first and second Hall ICs 20a and 20b is small, and the range of non-detection of rotation angle by the first and second Hall ICs 20a and 20b is large. For this reason, the rotation angle of the magnet 21 from when the second Hall IC 20b is switched from OFF to ON until when the first Hall IC 20a is switched from ON to OFF is 5°, whereas the rotation angle of the magnet 21 from when the second Hall IC 20b is switched from ON to OFF until when the first Hall IC 20a is switched from OFF to ON is 25°, and these rotation angles are different from each other.
In this type of rotation sensor 15, it is ideal that the ranges of detection and non-detection of rotation angle by the first and second Hall ICs 20a and 20b be equal in order to accurately detect the rotation angle. However, if the ranges of detection and non-detection of rotation angle by the first and second Hall ICs 20a and 20b are different, a variation in detected angle is produced, and it is difficult to detect the rotation angle with a high degree of accuracy.
A variation in rotation angle detected by the rotation sensor 15 is also caused, for example, by variations in the installation positions of the first and second Hall ICs 20a and 20b. For example, as shown in FIG. 13, when the reference angle to be detected by the first and second Hall ICs 20a and 20b is X°, the rotation angle actually detected by the first and second Hall ICs 20a and 20b is X°±α°. Here, X° is the reference angle, and α° is a variation. As a result, the detection accuracy of the rotation sensor 15 decreases. So, in the seat belt retractor 3 described in Japanese Unexamined Patent Application Publication No. 2009-113718, for example, the variations in the installation positions of the first and second Hall ICs 20a and 20b are minimized so that the detection accuracy of the rotation sensor 15 is maintained at least within the range of practical use. However, the variation in the rotation angle detected by the rotation sensor 15 is at almost the same level.
However, in the seat belt retractor 3 described in Japanese Unexamined Patent Application Publication No. 2009-113718, the rotating disk 18 (that is, the magnet 21) of the rotation sensor 15 is provided rotatably integrally with the rotating shaft 10a of the spool 10. That is, the rotation speed of the spool 10 and the rotation speed of the magnet 21 are equal or almost equal. For this reason, the rotation angle of the magnet 21 to be detected is small relative to a predetermined number of times of switching of the current polarities of the detection signals from the first and second Hall elements 20a and 20b. When the rotation angle of the magnet 21 to be detected is small, the variation in the rotation angle detected by the rotation sensor 15 has an influence on the rotation angle of the magnet 21 to be detected. In the seat belt retractor 3 described in Japanese Unexamined Patent Application Publication No. 2009-113718, the detection accuracy of the rotation sensor 15 is enough for practical use, but it is desirable to further improve the detection accuracy of the rotation sensor 15.
On the other hand, a conventional seat belt retractor employing a rotation sensor 15 that shows a variation as described above has room for improvement in controlling the belt tension with a high degree of accuracy by performing the retraction and withdrawal of the seat belt by the spool with a high degree of accuracy. A conventional seat belt apparatus having this conventional seat belt retractor has room for improvement in controlling the belt tension with a high degree of accuracy according to the running state of the vehicle, the usage state of the seat belt apparatus, and the like.